1. Field of the Invention
This invention relates to a Hydrogen-oxygen fuel cell.
2. Description of the Related Art
Hydrogen-oxygen fuel cells are known as a power generating system in which the sole reaction product is in principle water, thereby exerting little undesirable influences on the environment of the earth. In particular, research into fuel cells which have a solid polymer electrolyte made of perfluorosulfuric acid cation-exchange resin is greatly advancing in recent years. Consequently, fuel cells of this type are achieving compact and higher output, and are expected to be put into practical application as the power source of motor vehicles.
The electrolyte membrane of polymer electrolyte fuel cell (PEFC) is made generally of a film of proton-conducting ion exchange resin having a thickness between 50 and 200 xcexcm In particular, an ion exchange membrane made of a perfluorocarbon polymer having sulfuric acid groups is known to have excellent fundamental properties and is widely investigated. Japanese Patent Disclosure (Kokai) No. 8-119612 discloses an inorganic proton-conducting film made of unhydrated molybdic acid, which has greater durability than the polymer electrolyte membrane.
The polymer electrolyte fuel cell has gas diffusing electrode layers on both surfaces of the electrolyte membrane. Electric power is generated by the reaction between a gaseous fuel such as hydrogen or a liquid fuel such as methanol supplied to one of the electrode layer and oxidizers such as oxygen or air to the other electrode layer. In order to improve the output of this type of fuel cell, enhancement of the catalytic activity of the electrode, improvement of gas-diffusion electrode properties, reduction of ohmic losses, and suppression of electrolyte membrane degradation etc. have been attempted. The ohmic losses include losses due to the resistance of conductive materials, contact resistance and film resistance. The degradation of the electrolyte membrane includes the physical destruction of the membrane due to the vaporization of water in the electrolyte membrane and the increase of membrane resistance.
On the other hand, attempts are also made to make fuel cell systems smaller in order to employ fuel cells as a long life power source of portable equipment. In such fuel cell systems, size reduction of attachments including humidifiers and reformers are being considered.
The resistance of the ion exchange membrane tends to become smaller when the water content therein is higher, the concentration of the ion exchange group in the membrane is higher or the membrane thickness is smaller. The water content of the membrane changes with the humidity of supplied gases while the concentration of the ion exchange group of commercially available ion exchange membranes is limited. Hence it is sometimes attempted to reduce the losses due to membrane resistance by a relatively simple way of employing the membranes having smaller thickness.
However, when the thickness of the ion exchange membrane is reduced, the life of the membrane will become shorter because the strength of the membrane itself and the water content in the membrane are reduced. Japanese Patent Publication No. 5-75835 and No. 6-10277 disclose a process for improving the size stability and mechanical strength of an ion exchange membrane in the case when the membrane is made thinner to reduce the membrane resistance, wherein a reinforced ion exchange membrane is produced by impregnating a tetrafluoroethylene (PTFE) film with perfluorocarbon ion exchange resin. However, this process yields only unsatisfactory results.
In case a solid polymer ion exchange membrane is employed as the electrolyte membrane, it is known that proton migration from the fuel electrode (anode) to the oxidant electrode (cathode) is accompanied by the migration of water contained in the membrane. Moreover, heat is generated by the reaction in which protons are formed from the fuel gas on the fuel electrode, thereby causing local reduction of the mechanical strength of the electrolyte membrane gradually during long-term operation. This leads to the degradation of the electrolyte membrane, thus decreasing the power output of the fuel cell.
Generally, fuel cells which employ polymer ion exchange membranes as the electrolyte membranes thereof have a means to maintain the moisture of the electrolyte membranes in order to suppress the degradation of the electrolyte membranes. An example of such a means is a device to humidify fuel so that the moisture of the electrolyte membrane is adequately controlled. However, such a moisture maintaining mechanism occupies an excessively large volume compared with the fuel cell itself, thereby hindering the manufacture of more compact fuel cells.
Accordingly, it is an object of the present invention is to provide a fuel cell which can be made smaller in size by simplifying the moisture control of the electrolyte membrane.
On the other hand, fuel cells in which liquid or gaseous methanol is supplied directly to the fuel electrode without using a reformer have such problems that part of methanol to be used as fuel permeates during the fuel cell operation into the electrolyte membrane, thereby causing the swelling of the electrolyte membrane, or that part of methanol reaches the oxidant electrode and reacts directly with the oxidizer, thus causing the ineffective burning problem of the fuel, which is called methanol crossover.
Thus it is another object of this invention to provide a fuel cell having improved output performance wherein the degradation of the electrolyte membrane is suppressed by inhibiting the permeation of the liquid fuels such as methanol into the electrolyte membrane.
It is yet another object of this invention to provide a fuel cell of high performance and a small-size by forming proton-conducting connection paths within the catalyst layer in electrodes.
According to sections, each power generation section comprising a fuel electrode, an oxidant electrode, and an electrolyte membrane interposed between the fuel electrode and the oxidant electrode;
wherein the present invention, there is provided a fuel cell which comprises; a unit cell comprising a plurality of stacked power generation the electrolyte membrane consists of a laminate comprising a film made of a proton-conducting organic compound, and a film made of a proton-conducting inorganic glass provided at least on the surface of the film of the proton-conducting organic compound facing the anode.
In a preferred embodiment of the fuel cell of this invention, the films made of a proton-conducting inorganic glass are provided on both surfaces of the film of the proton-conducting organic compound, wherein the films of the proton-conducting inorganic compound have a porous structure, the pores on the surface contacting the fuel electrode preferably having a smaller average dimension than the pores on the surface contacting the oxidant electrode.
According to the present invention, there is further provided a fuel cell which comprises; a unit cell comprising a plurality of stacked power generation sections, each power generation section comprising a fuel electrode, an oxidant electrode, and an electrolyte membrane interposed between the fuel electrode and the oxidant electrode;
wherein the electrolyte membrane consists of a film made of a proton-conducting inorganic glass.
In a preferred embodiment of the fuel cell of this invention, the electrolyte membrane consisting of a film made of a proton-conducting inorganic glass has a larger number of pores on the surface contacting the oxidant electrode than on the surface contacting the fuel electrode.
According to the present invention, there is further provided a fuel cell which uses liquid fuels for power generation and comprises; a unit cell comprising a plurality of stacked power generation sections, each power generation section comprising a fuel electrode, an oxidant electrode, and an electrolyte membrane interposed between the fuel electrode and the oxidant electrode;
wherein the catalytic layers of the fuel electrode and oxidant electrode contacting the electrolyte membrane have a plurality of catalyst particles and proton-conducting particles, which catalyst particles are connected electrically by the proton-conducting particles.
In a preferred embodiment of the fuel cell of this invention, the proton-conducting particles are inorganic proton-conducting particles.
In a preferred embodiment of the fuel cell of this invention, the electrolyte membrane is made of an inorganic proton-conducting glass.